Well take the advise of a pro... The correct way to soundproof the room is to apply the gipsum board and then use ply to cover it... Actually gipsum board should be applied on both the sides of the wall to sound proof it... Also a carpet in your room will help a lot... Now as for the roof do a flase sealing.. And wolla you got a perfect sound proof room... If you really want to get it done (Trust me its expensive) then give me a call... I will handle it from there...